The atomic force microscope, hereinafter AFM, is an instrument capable of resolving surface detail down to the atomic level. The AFM provides topographic images by scanning a sharp stylus over a substrate surface. The microscope's tip, ideally terminating in a single atom, traces the contours of a surface with atomic resolution. In one mode of operation the tip is maneuvered to be in contact with the surface of a conducting or nonconducting substrate so that the atom at the probe tip senses the van der Waals interaction of the nearest atom of the sample. An image is generated by sensing the force on the tip as it is rastered over the surface. The instrument can be operated in one of two ways. Either the deflection of the cantilever tip is monitored as it interacts with surface features, or the cantilever tip deflection is kept constant and surface features are mapped by movement of the z piezoelectric element. The magnitude of the force applied by the tip to the sample can be estimated from the spring constant of the cantilever and the measured deflection. The image obtained by either mode of operation is a topographical map of the surface.
Further detail on the structure and operation of the AFM is disclosed in U.S. Pat. No. 4,724,318 of Binnig et al. issued Feb. 9, 1988; Wickramasinghe, H. K., Scientific American, October (1989) pp. 98-105; and Hansma et al., Science 242, October 14, 1988, pp. 209-216. The AFM has been used to image a wide variety of substances. See for example, Gould et al., J. Vac. Sci. Technol. A, 8(1), 369-373 (1990) and Ruger et al., Physics Today, 23-30, Oct. (1990).
The AFM is useful not only for the imaging or characterization of surfaces, but also for manipulating surfaces on a scale as small as subnanometers. Lithography using the AFM is of current interest in the area of electronics for information storage and in the design of transistors, diodes, integrated circuits and the like. The miniaturization of electronic components is increasing the speed of computers. The ability to manipulate single atoms or molecules with the AFM provides unique potential applications in microelectronics. Various approaches have been explored in the use of AFM for etching or writing.
Writing using the AFM wherein the microscope tip physically touches, scratches, indents, or creates holes in the substrate surface has been taught by Weisenhorn et al , Biophys. J., Vol. 58, pp. 1251-1258, Nov. (1990). Surface-induced polymerization of actin filaments occurred, but a low force limitation for nondestructive imaging was found. The filaments could be manipulated or removed from the surface by the tip of the AFM.
Another approach to writing with the AFM has been to use high forces for surface rearrangement of the atoms already present. Weisenhorn et al., Science, Vol. 247, pp. 1330-1333, Mar. 16, 1990, disclose rearrangement of tert-butyl ammonium ions on a zeolite surface with the AFM tip by application of a sufficiently large force.
In the known methods of lithography using the AFM, the image cannot be viewed simultaneously with its creation, but is viewed afterwards. A post writing treatment is often required to stabilize the image. The substrate or writing surface is destructively deformed using many of the known techniques. Further the known techniques are not sufficiently reliable or reproducible to use in manufacturing applications.
It is therefore an object of the present invention to provide a process for etching using the AFM wherein the etched image can be viewed simultaneously with its creation.
It is a further object of the present invention to provide a lithography process using the AFM which requires no post writing treatment to stabilize the image.
It is a further object of the present invention to provide a lithography process using AFM which can controllably remove one molecular layer at a time.
It is a further object of the present invention to provide a process for etching using the AFM which does not destructively deform the substrate.
It is a further object of the present invention to provide a process for etching or writing using the AFM which is reliably reproducible.
It is a further object of the present invention to provide a process for etching or writing using the AFM wherein the depth of the etched structures are known to a precision of (0.0003.times.n) nanometer, where n is the number of layers etched, and the bottom of the etched structures are atomically flat.
It is a further object of the present invention to provide a process for etching which can be controlled by controlling the force applied to the cantilever.